This application is based on Japanese patent application NO. 2002-163645, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a tunable filter to be used when selecting from, eliminating out of, or adding to a wavelength division multiplex optical signal a particular wavelength, mainly in an optical communication network in which wavelength division multiplexing is employed, and manufacturing method thereof, and further to an optical switching device comprising such tunable filter.
2. Description of the Related Art
Recently, wavelength division multiplexing has come to be popularly used in a field of optical communication, making it more and more important to select and pick up a particular wavelength out of a wavelength division multiplex (hereinafter referred to as xe2x80x9cWDMxe2x80x9d) optical signal. An optical add/drop device and an optical cross-connect device, which are currently available in the market, are generally designed to select a fixed wavelength out of a WDM optical signal. In such fixed wavelength filters, generally a Fabry-Perot etalon filter comprising a dielectric multi-layer film is incorporated because of its excellent cost performance.
Also, the U.S. Pat. No. 4,825,262 discloses a variable wavelength type Fabry-Perot etalon filter that can vary a wavelength by controlling a voltage. Further, the U.S. Pat. No. 5,739,945 discloses a Fabry-Perot etalon filter having a semi conductor/air constitution based on MEMS (Micro Electro-Mechanical System) technology, for increasing a resolution for selecting a wavelength.
Further, Japanese Patent Laid Open Publication (JP-A) H11-142752 discloses an interference filter of a variable transmitted wavelength type. In this variable transmitted wavelength type interference filter, a pair of substrates is disposed in parallel and a multi-layer film is formed on the respective confronting surfaces of the substrates with a certain clearance therebetween, which clearance is variable by an external force.
FIG. 11 shows a conventional tunable filter similar to the variable wavelength interference filter set forth in the foregoing literature. An optical multilayer film 1003 is formed on a substrate 1001, which is a silicon substrate. The optical multilayer film 1003 is composed of films of a high refractive index and films of a low refractive index, alternately layered. The high index film consists of a Ta2O5 film while the low index film consists of an SiO2 film, both of which are layered by ion beam deposition. Further an Au film is formed as an electrode 1004a on the optical multilayer film 1003 by sputtering.
In a manufacturing method of such tunable filter, a polyimide film (not shown) is first deposited to form a sacrificial layer, so that a cavity gap 1006 is formed by sacrificial layer etching. An Au film is then formed as an electrode 1004b on the polyimide film by sputtering. Then an Si3N4 film is formed by sputtering, which serves as a spring portion 1005. Further, another optical multilayer film 1002 is formed in a similar way to the optical multilayer film 1003, i.e. by forming alternate layers of Ta2O5 films used as high index film and SiO2 films used as low index film utilizing ion beam deposition method. Also, a portion on the substrate 1001 where a transmitted light beam 1009 is to go through is selectively eliminated by silicon crystal anisotropic etching utilizing KOH. And then the sacrificial layer is removed by oxygen plasma ashing, so as to form the cavity gap 1006.
In a tunable filter as constituted above, a clearance between the optical multilayer film 1002 and the optical multilayer film 1003 (cavity gap 1006) can be electrostatically varied by applying a voltage between the electrodes 1004a and 1004b. By controlling the cavity gap 1006 in this way, a wavelength that matches the cavity gap out of the WDM light 1007 can be selectively transmitted, so that a transmitted light beam 1009 is obtained. The remaining portion of the WDM light is reflected and turns into reflected light 1008. Consequently, only light of a desired wavelength passes through the filter, and the wavelength to be transmitted can be varied by controlling the cavity gap 1006.
As a result of performance measurement of a tunable filter of the conventional constitution as above, bandwidth for xe2x88x921 dB was 0.20 nm, adjacent channel cross talk value was xe2x88x9210.5 dB, tuning range was 1520 to 1630 nm, and insertion loss was 2.1 dB.
The foregoing conventional tunable filter may be reasonably applicable to a particular condition of use within a certain restriction, however in view of the expected increase of extent of wavelength division multiplication and demand for a more extensive flexibility of wavelength to be used, its performance is not sufficient. Specifically, for example, the adjacent channel cross talk must be smaller in order to increase a number of available wavelengths. Also it is essential to quickly change a wavelength to be added or dropped in the optical add/drop device.
In a conventional system in which a fixed wavelength filter is used, flexibility of wavelength to be used is virtually nonexistent and the filter has to be replaced as a whole in order to change a wavelength to be used, which is a great disadvantage as it requires considerable time and cost.
Also, the tunable filters disclosed in the U.S. Pat. No. 4,825,262 and U.S. Pat. No. 5,739,945 have a large adjacent channel cross talk value mainly because the Fabry-Perot etalon has only one cavity, therefore it is impossible to increase the number of available wavelengths, which is another serious disadvantage.
The present invention has been achieved in view of the foregoing problems, with an object to provide a tunable filter that can minimize the adjacent channel cross talk despite an increase of a number of available wavelengths and quickly switch a wavelength to be used, and manufacturing method thereof, and also an optical switching device comprising such tunable filter.
A tunable filter according to a first aspect of the present invention comprises a first substrate; a second substrate; a first and a fourth optical multilayer films respectively formed on confronting surfaces of the first substrate and the second substrate; a second optical multilayer film disposed with a clearance from the first optical multilayer film and supported by the first substrate through an elastically deformable spring portion; a third optical multilayer film disposed between the second optical multilayer film and the fourth optical multilayer film with a clearance from both of them and supported by the second substrate through an elastically deformable spring portion; a supporting member for mutually supporting said first substrate and said second in such a manner that a clearance between said first substrate and said second substrate can be changed; and a first, a second, and a third driving devices for respectively changing the clearance between the first and the second optical multilayer films, the clearance between the third and the fourth optical multilayer films and the clearance between the first substrate and the second substrate.
In such tunable filter, the first through the third driving devices may, for example, be electrostatic driving devices comprising a first pair of electrodes respectively provided on confronting surfaces of the first and the second optical multilayer films; a second pair of electrodes respectively provided on confronting surfaces of the third and the fourth optical multilayer films; and a third pair of electrodes respectively provided on the confronting surfaces of the first and the second substrates; for controlling the clearance between the first and the second optical multilayer films, the clearance between the third and the fourth optical multilayer films, and the clearance between the first substrate and the second substrate by changing a voltage to be applied to the respective electrodes.
Also, the first through the third driving devices may be electromagnetic driving devices comprising a first set of a coil film and a magnetic material film respectively provided on the confronting surfaces of the first and the second optical multilayer films; a second set of a coil film and a magnetic material film respectively provided on the confronting surfaces of the third and the fourth optical multilayer films; and a third set of a coil film and a magnetic material film respectively provided on the confronting surfaces of the first and the second substrates; for controlling the clearance between the first and the second optical multilayer films, the clearance between the third and the fourth optical multilayer films, and the clearance between the first substrate and the second substrate by changing a current value to be supplied to the respective coil films.
Also, the first through the third driving devices may be piezoelectric driving devices comprising a first piezoelectric device provided between the confronting surfaces of the first and the second optical multilayer films; a second piezoelectric device provided between the confronting surfaces of the third and the fourth optical multilayer films; and a third piezoelectric device provided between the confronting surfaces of the first and the second substrates; for controlling the clearance between the first and the second optical multilayer films, the clearance between the third and the fourth optical multilayer films, and the clearance between the first substrate and the second substrate by changing a voltage to be applied to the respective piezoelectric devices.
Also, the first through the third driving devices may be a combination of two or three ones selected out of a group including an electrostatic driving device for controlling the clearance between objects to be driven by changing a voltage to be applied between a pair of electrodes, an electromagnetic driving device for controlling the clearance between the objects to be driven by changing a current value to be supplied to a coil film in the sets of the coil film and a magnetic material film, and a piezoelectric driving device for controlling the clearance between the objects to be driven by changing a voltage to be applied to a piezoelectric device provided between the objects to be driven.
A tunable filter according to a second aspect of the invention comprises a first substrate; a second substrate; not less than three optical multilayer films disposed between confronting surfaces of the first substrate and the second substrate; a plurality of supporting members for mutually supporting the respective pairs consisting of adjacent ones among the first substrate, the second substrate, and the optical multilayer films in such a manner that a clearance between the adjacent ones can be controlled; and a plurality of driving devices for controlling a clearance between the adjacent optical multilayer films and a clearance between the first and the second substrates.
A tunable filter according to a third aspect of the invention comprises a substrate; a first optical multilayer film formed on a surface of the substrate; a plurality of second optical multilayer films disposed with a clearance from the first optical multilayer film and between one and another and respectively supported by the substrate through an elastically deformable spring portion; and a plurality of driving devices for controlling the clearance between adjacent ones among the first optical multilayer film and the second optical multilayer films.
In these tunable filters, the driving devices may, for example, be a one selected out of a group including an electrostatic driving device for controlling the clearance between objects to be driven by changing a voltage to be applied between a pair of electrodes, an electromagnetic driving device for controlling the clearance between the objects to be driven by changing a current value to be supplied to a coil film in the sets of the coil film and a magnetic material film, and a piezoelectric driving device for controlling the clearance between the objects to be driven by changing a voltage to be applied to a piezoelectric device provided between the objects to be driven.
Also, the driving devices may be a combination of two or three ones selected out of a group including an electrostatic driving device for controlling the clearance between objects to be driven by changing a voltage to be applied between a pair of electrodes, an electromagnetic driving device for controlling the clearance between the objects to be driven by changing a current value to be supplied to a coil film in the sets of the coil film and a magnetic material film, and a piezoelectric driving device for controlling the clearance between the objects to be driven by changing a voltage to be applied to a piezoelectric device provided between the objects to be driven.
Method of manufacturing a tunable filter according to a fourth aspect of the invention comprises the steps of forming a first optical multilayer film on a first substrate; forming a first sacrificial layer on the first optical multilayer film; forming a second optical multilayer film on the first sacrificial layer; forming a first spring portion that connects the second optical multilayer film and the first substrate; removing the first sacrificial layer by etching thus to form a gap between the first and the second optical multilayer films; forming a fourth optical multilayer film on a second substrate; forming a second sacrificial layer on the fourth optical multilayer film; forming a third optical multilayer film on the second sacrificial layer; forming a second spring portion that connects the third optical multilayer film and the second substrate; removing the second sacrificial layer by etching thus to form a gap between the third and the fourth optical multilayer films; and combining the first substrate and the second substrate with a clearance between the second optical multilayer film and the third optical multilayer film; and providing a first driving device between the first and the second optical multilayer films; providing a second driving device between the third and the fourth optical multilayer films; and providing a third driving device between the first and the second substrates.
Method of manufacturing a tunable filter according to a fifth aspect of the invention comprises the steps of forming a first optical multilayer film on a substrate; forming a plurality of layered composites including sacrificial layers and second optical multilayer films on the first optical multilayer film and a plurality of spring portions respectively connecting each of the second optical multilayer films with the substrate; removing the sacrificial layers by etching thus to form a gap between adjacent ones among the first and the second optical multilayer films; and providing a driving device between the adjacent ones among the first and the second optical multilayer films respectively.
Further, an optical switching device according to a sixth aspect of the invention comprises the foregoing tunable filters.
The tunable filter according to the invention is a tunable filter having a Fabri-Perot etalon structure, comprising not less than two cavity gaps. And as driving method for controlling or changing the clearance of the cavity gap, for example electrostatic drive, electromagnetic drive or piezoelectric drive, or a combination of two or three out of the electrostatic drive, electromagnetic drive and piezoelectric drive is adopted.
Also, in manufacturing method of the tunable filter having a Fabry-Perot etalon structure, the cavity gaps are formed by removing the sacrificial layers by etching.
Further, these tunable filters can be used in an optical switching device of an optical cross-connect device or an optical add/drop device, etc.
The objects are achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the present invention.
This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.